This invention concerns the field of filtration and more specifically, filtration using rotary disc devices containing filters.
Filtration devices are used to separate one or more components of a fluid from other components. Common processes carried out in such devices include classic filtration, microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, pervaporation, water splitting, sieving, affinity separation, affinity purification, affinity sorption, chromatography, gel filtration, and bacteriological filtration. As used herein, the term "filtration" includes all of those separation processes as well as any other processes using a filter that separate one or more components of a fluid from the other components of the fluid.
Filtration processes make use of the greater filter permeability of some fluid components than others. As used herein, the term "filter" includes any article made of any material that allows one or more components of a fluid to pass through it to separate those components from other components of the fluid. Thus, the term "filter" includes metallic and polymeric cloth filters, semipermeable membranes and inorganic sieve materials (e.g., zeolites, ceramics). A filter may have any shape or form, for example, woven or non-woven fabrics, fibers, fiber bundles, membranes, sieves, sheets, films, rods, and combinations thereof.
The components of the fluid that pass through the filter comprise the "permeate" and those that do not pass (i.e., are rejected by the filter or are held by the filter) comprise the "retentate." The valuable fraction from the filtration process may be the retentate or the permeate or in some cases both may be valuable.
A common problem in all filtration devices is blinding or clogging of the filter. Permeate passing through the filter from the fluid layer adjacent to the feed side of the filter leaves a retentate layer adjacent to or on that side of the filter having different composition than that of the bulk feed
a fluid. This material may bind to the filter and clog its pores (that is, foul the filter) or remain as a stagnant boundary layer, either of which hinders transport of the components trying to pass through the filter to the permeate product side of the filter. In other words, the mass transport rate through the filter per unit membrane area (i.e., flux) is reduced and the inherent sieving capability of the filter is adversely affected.
Generally, fouling of the filter is chemical in nature, involving chemisorption of substances in the feed fluid onto the filter's internal (pore) and external surface area. Unless the chemical properties of the filter surface are altered to prevent or reduce adsorption, frequent and costly filter replacement or cleaning operations are necessary.
One of the most common causes of fouling arises from the low surface energy (e.g., hydrophobic nature) of many filters. U.S. Pat. Nos. 4,906,379 and 5,000,848, which are commonly assigned with this application, disclose chemical modification to increase the surface free energy (e.g., hydrophilicity) of filter surfaces. In general, however, relatively little attention has been given to modifying surface chemistry to reduce filter fouling. (U.S. Pat. Nos. 4,906,379 and 5,000,848 and all of the other patent documents, publications, and other documents cited in this application are incorporated herein by reference in their entireties for all purposes.)
In contrast to the chemical nature of most fouling problems, the formation of a boundary layer near the surface of the filter is physical in nature, arising from an imbalance in the mass transfer of feed fluid components towards the filter surface as compared to the back-transfer from the boundary layer to the bulk feed fluid. Some form of force (for example, mechanical, electro-kinetic) must be used to promote the desired mass transfer away from the filter surface. Unfortunately, few strategies have been developed that promote adequate back-mixing to reduce the boundary layer or prevent its formation.
The most common strategy is called "cross-flow" filtration ("CFF") or "tangential flow" filtration ("TFF"). In principle, the feed fluid is pumped across (i.e., parallel to) the outer surface of the filter at a velocity high enough to disrupt and back-mix the boundary layer. In practice, however, cross-flow has several disadvantages and cannot be used at all in certain cases.
A different approach to eliminating the stagnant boundary layer involves decoupling the feed flow rate from the applied pressure. With this approach, a structural element of the filtration device, rather than the feed fluid, is moved to effect back-mixing and reduction of the boundary layer. The moving body may be the filter itself or a body located near the filter element.
Filter devices utilizing movement of a structural element/mechanical agitation are known. In the rotary filtration devices of commonly assigned U.S. Pat. Nos. 4,790,942, 4,867,878, 4,876,013, 4,911,847, and 5,008,848, at least one element defining the fluid filtration gap moves with respect to another element that also defines the gap. For example, one cylinder rotates within another cylinder to create hydrodynamic Taylor vortices in the fluid in the annular gap between the cylinders. Those vortices help reduce or eliminate the stagnant boundary layer near the surface of a filter located on a cylindrical surface defining the filtration gap.
In U.S. Pat. No. 4,216,094 a sector body for use in a rotary disc dewatering device has radially extending grooves. Any slackness in the filter medium fabric covering the sector body is gathered into the radial grooves.
U.S. Pat. No. 4,330,405 concerns a rotating vacuum disc filter divided into segments by means of grooved radial arms that are designed to receive caulking media for holding a filter cloth on each of the segments.
U.S. Pat. No. 4,376,049 concerns a rotary apparatus having a circular design. Centrifugal forces cause permeate to flow from the center of the rotor plate through the rotating filter elements. The vanes on the rotor plate holding the filter elements are curved rearwardly with respect to the forward direction of rotation. The curvature helps retain the filter elements in place and enhances the pumping capability of the system as the rotor is rotated in the forward direction. The filter elements eventually become clogged or packed with material and must be replaced.
U.S. Pat. No. 4,708,797 concerns a rotary disc filter element having supporting mesh between the bottom plate and the filter mesh. The bottom plate is stiffened by corrugations, which extend radially and coaxially of the hub.
U.S. Pat. No. 4,781,835 concerns a disc concentrator in which filter chambers composed of two adjacent filter discs are attached to a drum-like filtrate collection chamber and are adapted to rotate therewith. The filtrate passes through the filter discs and enters the collection chamber, from which the filtrate flows into a collection space.
U.S. Pat. No. 4,950,403 concerns a screen segment for rotary disc filters or thickeners.
U.K. Pat. No. 1,057,015 concerns a rotary filtration device for dynamic filtration of slurries in which a filtration body carrying the filter rotates. A stationary member inside the rotating filtration body may be used to help discharge the permeate that collects inside the filtration body.
In U.S. Pat. No. 3,477,575 one set of filter elements is mounted on a common shaft for rotation. A second set of filter elements is fixed on the inner wall of the device, and the rotatable and stationary filter elements are interleaved to create a serpentine flow path along which the feed fluid flows. Permeate is withdrawn through both sets of filters.
This type of filtration device was commercialized by the Swiss company Willi Bachofen. Because of the interleaving of the two sets of filter elements and because of the location of the feed fluid inlet and outlet, a substantial portion of the feed fluid flow is generally parallel to the major surfaces of the filters. Thus, although this device uses rotation of mechanical elements (one set of filters), it also uses cross-flow to help reduce stagnation at the fluid/filter interface.
Historically, such rotary filters have been used at relatively low rotation rates to dewater suspensions in so-called cake filtration (see Schweigler and Stahl, "High Performance Disc Filter for Dewatering Mineral Slurries," Filtration and Separation, January/February, pages 38-41 (1990)). See also Ingersoll-Rand, "Upgrade your entire filtering and/or washing operation with the new Artisan Dynamic Thickener/Washer," Bulletin No. 4081, 4 pages (2/86); and Ingersoll-Rand, "Patented filter/wash capability permits simultaneous washing and filtering," Bulletin No. 4060, 4 pages (8/83).
Another strategy for reducing the stagnant boundary layer adjacent to a rotating filter surface is to use high filter rotation rates, e.g., 1000 rpm. See Parkinson, "Novel Separator Makes Its Debut," Chemical Engineering (January 1989), 1-page reprint by Aqua Technology Resource Management, Inc.; Aqua Technology Resource Management, Inc., "How to Keep Your Fluid Processing Budget from Going to Waste," 3-page brochure; and Aqua Technology Resource Management, Inc., 4-page brochure (untitled) discussing "Technology Background," "Overcoming Concentration Polarization," etc. However, use of a rotating filter element is complicated by the need for adequate rotary sealing means to maintain the separation of feed fluid from permeate. Also, the rotating filter and sealing means need to be strong enough to withstand centrifugal forces.
An alternative disc filter design utilizes a stationary filter and a closely spaced non-filter rotating element. Wronski, Molga, and Rudniak, "Dynamic Filtration in Biotechnology," Bioprocess Engineering, Volume 4, pages 99-104 (1989), report testing such a device against a device that has a stationary filter and an oppositely disposed rotating disc filter, and against a cross-flow device, and against a rotating cylindrical filtration device. See also Wronski and Mroz, "Power Consumption in Dynamic Disc Filters," Filtration & Separation, November/December, pages 397-399 (1984); Wronski, Rudniak, and Molga, "Resistance Model for High-Shear Dynamic Microfiltration," Filtration & Separation, November/December, pages 418-420 (1989); Rudniak and Wronski, "Dynamic Microfiltration in Biotechnology," Proceedings 1st Event: Bioprocess Engineering, Institute of Chemical and Process Engineering, Warsaw University of Technology, Warsaw, Poland, June 26-30, 1989; Molga and Wronski, "Dynamic Filtration in Obtaining of High Purity Materials--Modelling of the Washing Process," Proceedings of the Royal Flemish Society of Engineers, Antwerp, Belgium, October 1988, Volume 4, pages 69-77; Wronski and Mroz, "Problems of Dynamic Filtration," Reports of the Institute of Chemical Engineering, Warsaw Techn. Univ., T.XI, Z. 3-4, pages 71-91 (1982); and Wronski, "Filtracja dynamiczna roztworow polimerow," Inz. i Ap. Chem., number 1, pages 7-10 (1983).
Murkes and Carlsson, Crossflow Filtration--Theory and Practice, 133 pages, John Wiley & Sons, New York (1988), particularly pages 22-26 and 69-125 and most particularly in Section 3.5 at pages 93-100, disclose that the flux of a stationary filter can be enhanced by rotating various elements near the stationary filter. The elements reported to have been tested include a cross, a double cross, a propeller, a spoke wheel, and discs either flat (plain) or having radial blades (vanes extending from the plane of the disc). See also Watabe, "Experiments on the Fluid Friction of a Rotating Disc with Blades," Bulletin of JSME, Volume 5, number 17, pages 49-57 (1962); and Shirato, Murase, Yamazaki, Iwata, and Inayoshi, "Patterns of Flow in a Filter Chamber during Dynamic Filtration with a Grooved Disk," International Chem. Eng., Volume 27, pages 304-310 (1987).
Conventional rotating disc filter devices utilize stacked filter disc arrangements. Historically, most of these devices comprise disc filters that are rotated by a central drive shaft to which the filter elements are attached. Some rotating disc devices utilize stationary filter discs separated from each other by rotary elements attached to the shaft. Murkes and Carlsson, above, FIG. 3.15 at page 91. In this type of device the stationary filter element surrounds the central rotating drive shaft. Accordingly, to change filters requires disassembly of the device followed by sequential removal of rotors and filters from the stacked array. For example, to remove the nth filter requires removing n-1 rotors and filters. This obviously is a significant disadvantage of such a design because of the labor and downtime involved.
Devices in which the filter holding means is an integral part of the segmented vessel housing (e.g., containment vessel wall) also have drawbacks. See, e.g., Murkes and Carlsson, above, FIGS. 3.11 to 3.14 (pages 87-90). As in the previous case, to change the nth filter requires removal of n-1 segments as well as rotors. Also, scale-up is limited by engineering tolerances. Furthermore, adding more segments to increase the capacity of the device requires additional seals, thereby increasing the risk of device failure.
In conventional disc filters where either the permeate or the feed fluid is routed within the rotating shaft, seals must be provided to keep permeate from mixing with feed fluid. Seals can have no moving parts (stationary seals) or can have moving parts (mechanical seals, e.g., dynamic seals). Some types can act as either stationary or mechanical seals (for example, O-rings and face seals).
U.S. Pat. Nos. 4,025,425 and 4,132,649 concern purification devices each having a stack of filter or membrane packs. Each multilayer pack comprises membranes, filter paper, and support sheets. Each stack is rotated to provide centrifugal force that helps sweep away membrane-plugging material. Holes in the packs are aligned to provide conduits ("pipes") for fluid flow (e.g., concentrate removal). Stationary seals, namely, gaskets, separate feed/retentate from permeate.
U.S. Pat. No. 4,717,485 concerns a rotary separation device having a chamber with a plurality of porous filter discs capable of rotating around a common central passage. An inlet introduces fluid into the interior of the chamber near the central passage of the discs. Centrifugal force helps move any solids in the feed fluid towards the periphery of the discs and also forces filtrate or permeate, which is collected inside the discs after passing through the filters mounted thereon, to the periphery of the discs, where it is collected in a permeate collection manifold or header. Reference is made to "free rotational seals" (column 4, line 48) and to stationary seals (e.g., column 5, lines 3-6). Some type of stationary seal must also be used where the discs are connected to the permeate collection manifold.
However, mechanical seals typically exhibit at least some leakage and must be replaced at regular intervals. Lebeck, Principles and Design of Mechanical Face Seals, pages 17-20, 107, 146 (John Wiley & Sons, Inc. 1991); Wisniewski, "Anticipated Effects of Seal Interface Operating Conditions on Biological Materials," Bioprocess Engineering Symposium, The American Society of Mechanical Engineers (1989), pages 87-96; Todhunter, "Improving the Life Expectancy of Mechanical Seals in Aseptic Service," Bioprocess Engineering Symposium, The American Society of Mechanical Engineers (1989), pages 97-103; Fodor, "Mechanical Seals: Design Solutions for Trouble Free Sterile Applications," Bioprocess Engineering Symposium, The American Society of Mechanical Engineers (1990), pages 89-98; Snowman, "Sealing Technology in Lyophilizers," in Bioprocess Engineering Symposium, The American Society of Mechanical Engineers (1989), pages 81-86 Alternatively, fluidic magnetic seals, which have zero leakage, tolerate only small pressure differences and the ferro fluid used in the seal must be compatible with the process fluid. Lebeck, Principles and Design of Mechanical Seals, above, page 6. Accordingly, it is desirable to avoid such seals for separating permeate and feed fluid.
The effectiveness of rotating disc filter devices depends in large part upon the flowpaths of the feed, retentate, and permeate fluids. For example, the feed fluid may enter at the top of the device and pass downward adjacent to succeeding filter discs (see the diagram of a serially arranged device in FIG. 3.4 at page 78 of Murkes and Carlsson). Along such a flowpath, the feed must fight the natural outward pumping action of the discs at every other filter stage (i.e., at those stages in which the feed is supposed to flow from the circumference of the filter surface towards the center). Consequently, the potential for feed materials to deposit upon the filter surface is increased as the feed is concentrated and thereby becomes more viscous. Thus, the stages where the feed must flow against the outward pumping action of the disc will experience more deposition than the stages where the feed flows with the pumping action of the disc. Accordingly, performance of the filtration device will be non-uniform and will depend greatly on the viscosity and solids concentration of the feed.
Alternatively, a parallel feed arrangement, such as the one shown in the left side of FIG. 3.7 at page 81 of Murkes and Carlsson can be used to deliver the feed and recover retentate and permeate. However, with this scheme, the feed fights the natural outward pumping action of the rotating discs at every stage. Consequently, the performance of the entire device will be compromised as the feed solids content and viscosity increase. This decline in performance is further aggravated because at the same time the feed fluid/retentate is becoming more concentrated as it moves towards the shaft, that progressively concentrated fluid is entering zones of decreasing filtration efficiency (the cleaning forces on the filter decrease as the fluid moves from the circumference towards the shaft because local linear velocity decreases).
Means to overcome the potential for buildup of rejected species caused by the flowpath limitations discussed above may involve changing either the rotating disc design (e.g., adding blades or grooves), or changing the feed pathways, or both. Such pathways may involve hollow rotating shafts having ports (or nozzles) to direct the feed to either or both sides of the filter members (parallel arrangement). However, this approach weakens the shaft, increases its complexity, and hence increases its cost. Additionally, this porting arrangement may result in zones of high turbulence at the injection point. Such high shear zones are undesirable for shear-sensitive materials.
Despite all of these designs there is a continuing need for mechanically agitated disc filters that, among other things, allow unobstructed access to each filter element, enable relatively easy and rapid replacement of individual filter elements and minimize the time required for such operations, provide more reliable means for isolating (i.e., sealing) the permeate from the feed fluid/retentate, introduce feed fluid near the rotor shaft, permit permeate composition to be monitored for each filter, and allow individual filter elements to be isolated during operation (for example, in case of a leak in one of the filter elements) without having to shut down the entire device.